1. Field of the Invention
The invention relates generally to apparatus, systems, and methods for separating and collecting particulate matter from a fluid. More particularly, the invention relates to a wetted wall cyclone and method of using the same for separating and collecting particular matter on a liquid layer. Still more particularly, the invention relates to a wetted wall cyclone and method of using the same for bioaerosol collection and concentration.
2. Background of the Invention
A cyclone separator is a mechanical device conventionally employed to remove and collect particulate matter or fine solids from a gas, typically air, by the use of centrifugal force. The gaseous suspension containing the fine particulate matter, often referred to as an “aerosol,” is tangentially flowed into the inlet of a generally cylindrical cyclone body, resulting in a vortex of spinning airflow within the cyclone body. As the aerosol enters the cyclone, it is accelerated to a speed sufficient to cause the entrained particles with sufficient inertia to move radially outward under centrifugal forces until they strike the inner wall of the cyclone body.
In a wetted wall cyclone, the particulate matter moving radially outward is collected on a liquid film or layer that is formed on at least a portion of the inner surface of the cyclone wall. The liquid film is created by injecting the liquid into the air stream or into the cyclone body, where it is eventually deposited on the inner wall of the cyclone to form the liquid film. The liquid may be continuously injected or applied at periodic intervals to wash the inner surface of the cyclone wall. Shear forces caused by the cyclonic bulk airflow, which may be aided by the force of gravity, cause the liquid layer on the inner surface of the cyclone wall, as well as the particulate matter entrained therein, to move axially along the inner surface of the cyclone wall as a film or as rivulets towards a skimmer positioned downstream of the cyclone body. In wetted wall cyclone separators using water as the injected liquid, the suspension of water and entrained particulate matter is often referred to as a “hydrosol.”
The liquid film or rivulets on the inner surface of the cyclone wall including the entrained particulate matter are separated from the bulk airflow by a skimmer from which the liquid film and entrained particles are aspirated from the cyclone body. The processed or “cleansed” air (i.e., the air remaining after the particulate matter has been separated and collected) exits the cyclone body and may be exhausted to the environment or subject to further separation. In this manner, at least a portion of the particulate matter in the bulk airflow is separated and collected in a more concentrated form that may be passed along for further processing or analysis. The concentration of the particulate matter separated from the bulk airflow can be increased by several orders of magnitude by this general process.
Wetted wall cyclone separators are used for a variety of separating and sampling purposes. For instance, wetted wall cyclones may be used as part of a bioaerosol detection system in which airborne bioaerosol particles are separated and collected in a concentrated form that can be analyzed to assess the characteristics of the bioaerosol particles.
The effectiveness or ability of the cyclone separator to separate and collect such particulate matter is often measured by the aerosol-to-hydrosol collection efficiency which is calculated by dividing the rate at which particles of a given size leave the cyclone separator in the hydrosol effluent stream by the rate of at which particles of that same size enter the cyclone in the bulk airflow or aerosol state.
In some conventional wetted wall cyclone, the liquid skimmer is connected to the cyclone body at a location where the cyclone body has an expanded or increased radius section. In such a diverging flow region, the cyclonic airflow tends to decelerate in the axial direction. As a result, the hydrosol liquid flowing along the inner wall of the cyclone body proximal the skimmer may collect and buildup in a relatively stagnant toroidal-shaped mass or ring-shaped bolus. Some of the hydrosol contained within such a bolus may be swept up and entrained in the cyclonic airflow, and exit the cyclone body along with such separated airflow, thereby bypassing the skimmer and associated aspiration. This phenomenon, often referred to as “liquid carryover”, degrades the cyclone's separation and collection capabilities, and may significantly decrease the aerosol-to-hydrosol collection efficiency. For instance, Battelle Memorial Institute, Columbus, Ohio developed a wetted wall cyclone that was designed to operate at an air flow rate of 780 L/min and an effluent liquid flow rate of about 1.5 mL/min. The aerosol-to-hydrosol collection efficiency for particles in the size range of 1.5 to 6.5 μm aerodynamic diameter (AD) is about 60%; however, the unit frequently exhibits water carryover which significantly reduces the aerosol-to-hydrosol efficiency.
In some applications, it may be particularly desirable to control the temperature of the cyclone body, injected liquid, and hydrosol. For instance, the effectiveness of a wetted wall cyclone operated in a sub-freezing environment may be significantly reduced if the injected liquid and/or hydrosol begin to solidify or freeze. If the injected liquid and/or hydrosol begin to solidify, the ability to aspirate the hydrosol may become severely limited. As another example, for sampling bioaerosols, it is often preferred that the collected aerosol particles be preserved for subsequent analysis and study. The preservation of viability of biological organisms may necessitate a particular temperature range within the cyclone. However, many conventional wetted wall cyclones do not include any means or mechanism to control the temperature of the cyclone body, injected fluid, or hydrosol. The Battelle cyclone separator previously discussed employs an electric heating element to control the temperature of the cyclone body, however, it consumes relatively large amounts of power as the ambient temperature approaches and dips below freezing. For example, in environments having an ambient temperature below about −10° C., the Battelle cyclone requires about 350 watts of electrical power. Still further, the few conventional heated wetted wall cyclones generally employ a single heater to control the temperature of the cyclone body. However, due to the air flow patterns within the cyclone body, variations in local turbulent heat transfer coefficients arise, which can result in temperature gradients along the cyclone body. In heated wetted wall cyclones employing a single heat source, hot spots and/or cold spots tend to develop on the cyclone body. Such hot spots may damage biological particles in the liquid state, and further, cold spots may cause partial solidification of the injected liquid in certain regions of the cyclone body.
Accordingly, there remains a need in the art for wetted wall cyclone separators capable of operation in sub-freezing environments. Such a wetted wall cyclone separator would be particularly well received if it allowed for variable temperature control of select areas of the cyclone body, and offered the potential for reduced water carryover and improved efficiency.